The battery is not holding a full charge. Have the battery load tested and see if it's good. If the battery is good then you probably have an excessive amount of Ignition-Off Draw (IOD).
IOD is a normal condition where power is being drained from the battery with the ignition switch in the Off position. A normal vehicle electrical system will draw from five to thirty-five milliamperes (0.005 to 0.035 ampere) with the ignition switch in the Off position.
Up to thirty-five milliamperes are needed to enable the memory functions for the Powertrain Control Module (PCM), digital clock, electronically tuned radio, and other modules which vary depending on the vehicles optional equipment.
A IOD test will determine whether your car is drawing an excessive amount of IOD and draining the battery. Some of the things that can cause excessive IOD include:
² Electrical items left on.
² Inoperative or improperly adjusted switches.
² Inoperative or shorted electronic modules and components.
² An internally shorted generator.
² Intermittent shorts in the wiring.
Check the electrical connections under the front seats. It's not uncommon for these to come loose because of items rolling under seats, vacuuming under the seats, etc...